[#84482] Fix inconsistent force assignment (#6541)

Signed-off-by: Artur Bieniek <abieniek@internships.antmicro.com>

src/V3Force.cpp

@@ -147,7 +147,7 @@ private:
     //  AstVar::user1p        -> ForceComponentsVar* instance (via m_forceComponentsVar)
     //  AstVarScope::user1p   -> ForceComponentsVarScope* instance (via m_forceComponentsVarScope)
     //  AstVarRef::user2      -> Flag indicating not to replace reference
-    //  AstVarScope::user3p   -> AstNodeExpr*, the RHS expression
+    //  AstVarScope::user3p   -> AstAssign*, the assignment <lhs>__VforceVal = <rhs>
     const VNUser1InUse m_user1InUse;
     const VNUser2InUse m_user2InUse;
     const VNUser3InUse m_user3InUse;
@@ -197,11 +197,9 @@ public:
     ForceComponentsVarScope* tryGetForceComponents(AstVarRef* nodep) const {
         return m_forceComponentsVarScope.tryGet(nodep->varScopep());
     }
-    void setValVscpRhsExpr(AstVarScope* valVscp, AstNodeExpr* rhsExpr) {
-        valVscp->user3p(rhsExpr);
-    }
-    AstNodeExpr* getValVscpRhsExpr(AstVarScope* valVscp) const {
-        return VN_CAST(valVscp->user3p(), NodeExpr);
+    void setValVscpAssign(AstVarScope* valVscp, AstAssign* rhsExpr) { valVscp->user3p(rhsExpr); }
+    AstAssign* getValVscpAssign(AstVarScope* valVscp) const {
+        return VN_CAST(valVscp->user3p(), Assign);
     }
 };
 
@@ -249,9 +247,9 @@ class ForceConvertVisitor final : public VNVisitor {
         // Set corresponding value signals to the forced value
         AstAssign* const setValp
             = new AstAssign{flp, lhsp->cloneTreePure(false), rhsp->cloneTreePure(false)};
-        transformWritenVarScopes(setValp->lhsp(), [this, rhsp](AstVarScope* vscp) {
+        transformWritenVarScopes(setValp->lhsp(), [this, rhsp, setValp](AstVarScope* vscp) {
             AstVarScope* const valVscp = m_state.getForceComponents(vscp).m_valVscp;
-            m_state.setValVscpRhsExpr(valVscp, rhsp);
+            m_state.setValVscpAssign(valVscp, setValp);
             rhsp->foreach([valVscp, this](AstVarRef* refp) { m_state.addValVscp(refp, valVscp); });
             return valVscp;
         });
@@ -393,15 +391,15 @@ class ForceReplaceVisitor final : public VNVisitor {
             if (!m_state.getValVscps(nodep)) break;
             for (AstVarScope* const valVscp : *m_state.getValVscps(nodep)) {
                 FileLine* const flp = nodep->fileline();
-                AstVarRef* const valp = new AstVarRef{flp, valVscp, VAccess::WRITE};
-                AstNodeExpr* rhsp = m_state.getValVscpRhsExpr(valVscp);
-                UASSERT_OBJ(rhsp, flp, "RHS of force/release must be an AstNodeExpr");
-                rhsp = rhsp->cloneTreePure(false);
+                AstAssign* assignp = m_state.getValVscpAssign(valVscp);
+                UASSERT_OBJ(assignp, flp, "Missing stored assignment for forced valVscp");
 
-                ForceState::markNonReplaceable(valp);
-                rhsp->foreach([](AstVarRef* refp) { ForceState::markNonReplaceable(refp); });
+                assignp = assignp->cloneTreePure(false);
 
-                m_stmtp->addNextHere(new AstAssign{flp, valp, rhsp});
+                assignp->rhsp()->foreach(
+                    [](AstVarRef* refp) { ForceState::markNonReplaceable(refp); });
+
+                m_stmtp->addNextHere(assignp);
             }
             break;
         }

test_regress/t/t_force_struct_partial.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_force_struct_partial.v

@@ -0,0 +1,31 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+`define stop $stop
+`define checkh(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got='h%x exp='h%x\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0)
+
+typedef struct packed {
+    logic sig1;
+    logic sig2;
+    logic not_forced;
+} s1;
+
+module t(clk);
+    input clk;
+    s1 s1inst;
+    logic a = 1'b0;
+    logic b;
+    initial force s1inst.sig1 = a;
+    always @(posedge clk) begin
+        force s1inst.sig2 = 1'b1;
+        force s1inst.sig1 = b;
+
+        `checkh(s1inst.sig1, b);
+        `checkh(s1inst.sig2, 1'b1);
+
+        $finish;
+    end
+endmodule